The art of lubrication (reduction of friction and wear) is at least as ancient as the greasing of Egyptian and Roman chariot wheel axels with tallow. During the industrial revolution, use was made of animal and vegetable oils to lubricate primitive machinery. Over the years, what was once an empirical art for minimizing solid friction and reducing wear of rubbing surfaces has evolved into a sophisticated science.
The term "lubrication" is applied to two different fact situations. "Fluid lubrication", the first situation, occurs when a thick film of a liquid or gas completely separates two solids which are in relative movement to one another. The second is known as "solid lubrication". Solid lubrication arises when a film of solids, which may be of substantial thickness, is interposed between solid surfaces which are sliding or rolling-sliding relative to one another. This invention relates to "solid film lubrication" in which the film has specific compositional and frictional properties.
The most fundamental of frictional properties is the "coefficient of friction" which is the ratio between the friction (tangential) force and the normal (vertical) force acting between bodies in relative contact. In general, the friction force is independent of the velocity of relative motion. However, there are many exceptions.
It is well known that the friction force required to initiate sliding between two bodies (static friction) is usually greater than the force required to maintain sliding between the two bodies (dynamic friction). This has given rise to the concept that there are two coefficients of friction, namely--static (for surfaces at rest) and dynamic (for surfaces in motion).
Recent investigations have shown that this basic dichotomy is a gross oversimplification and that the static friction coefficient is a function of time of contact, whereas the kinetic (dynamic) friction coefficient is a function of velocity throughout the range of velocities between the two relatively moving bodies. The change in kinetic friction coefficient is appreciable if the relative sliding velocities are extremely low or extremely high or if there is rolling-sliding contact between the bodies. When friction increases with speed, it is characterized as having a positive friction, whereas when it decreases with speed, it is known as having a negative friction characteristic. Hereinafter, the term "positive friction" means that the coefficient of friction increases with speed of sliding and a "high" coefficient of friction is greater than 0.10.
With sliding systems with negative friction characteristic, friction oscillations may arise and thus the squeaking and chattering characteristic of many sliding systems is produced. This squeaking and chattering can be eliminated by
a) making the mechanical system or equipment, in which it occurs, very rigid; PA1 b) by reducing the friction between the relatively moving components to very low levels; or PA1 c) by changing the friction characteristic from a negative one to a positive one. PA1 (a) at least 20% by weight of a polymer medium; PA1 (b) at least 5% by weight of a solid lubricant; and PA1 (c) at least 5% by weight of a friction modifier so that the coefficient of friction between steel bodies in rolling-sliding contact lubricated using said composition is greater than 0.10 and increases with an increase in the relative speed of sliding movement between said bodies. PA1 (a) 20-90% by weight of a polymer medium; PA1 (b) 5-40% by weight of a solid lubricant producing a coefficient of friction preferably of about 0.06; PA1 (c) 5-40% by weight of powderized solid mineral friction modifier producing a coefficient of friction between steel bodies greater than 0.4; PA1 (a) about 40% by weight of a polyester resin media; PA1 (b) about 30% by weight of molybdenum disulphide solid lubricant producing a coefficient of friction between steel bodies of about 0.06; and PA1 (a) about 70% by weight of polyester resin media; PA1 (b) about 14% by weight of molybdenum disulphide solid lubricant; PA1 (c) about 14% by weight of powderized talc (magnesium silicate) PA1 (d) about 0.3% promoters to improve and speed up the cure rate and gel time and PA1 (e) about 1.7% initiators of polymerization such that the coefficient of friction produced between steel bodies is about 0.15 and increases directly with an increase in the speed of sliding.
National Research Council Technical Report--WE-47, entitled "A Low Velocity Friction Machine For The Studies Of Static and Dynamic Liquid Frictions--With Special References To The Evaluation Of Slide And Way Oils", P. L. Ko and J. T. Lowe, 1985/10 25 discloses certain liquid lubricants having positive friction characteristics with coefficients of friction below 0.17.
Durafilm Materials Corporation manufactures and sells anti-wear, anti-friction lubricant under the trade-mark DURAFILM SPC. This lubricant is designed to reduce wheel and rail wear on heavy-haul rail roads and rapid transit systems. DURAFILM SPC is applied directly to the wheel flange to reduce friction between the wheel flange and the gage face. The product is a combination of anti-wear and anti-friction agents which are suspended in a solid polymeric carrier. Friction of the carrier against the wheel flange activates the anti-wear and antifriction agents. The DURAFILM SPC product does not display positive friction characteristics.
A lubricant insert for lubricating rolling bearings is known in the art. The insert is composed of a firm, tough, solid gel matrix containing a lubricating oil of lubricating viscosity is known in the art. The oil exudes from the matrix to provide an oily surface in contact with the elements of the bearing. The preferred form of lubricant insert comprises an annular member having a plurality of circumferentially spaced axially directed projections which have surfaces conforming to the contour of the rolling elements. The projections are of a predetermined orientation to permit ease of assembly and locking of the insert in place in the annular space between the inner and outer rings of a bearing.
In a further prior art lubricant, the lubricating composition comprises a solid lubricant homogeneously dispersed throughout a wax-polymer matrix system, suitable for lubricating railroad center plates, and a method of lubricating a railroad center plate. The lubricant composition comprising a solid lubricant selected from the group consisting of molybdenum disulphide, graphite, babbit alloy, lead, copper, red lead, zinc oxide, powdered zinc and talc, homogeneously dispersed throughout a petroleum wax-polymer system. This lubricating composition has a Coefficient of friction below 0.1 and is not positive.
Many steel rail-wheel transportation systems including freight, passenger and mass transit systems suffer from squealing or other types of high noise levels which cause a nuisance to persons dwelling close to such systems. The origin of such squealing can be traced to the negative friction characteristic between an unlubricated steel wheel and a steel rail. In any dynamic steel wheel-steel rail system, there is a constantly moving zone of contact. For purposes of discussion and analysis, it is convenient to treat the zone of contact as stationary while the rail and wheel move through the zone of contact. When the rail moves through the zone of contact at exactly the same speed and direction as the cylindrical wheel, the wheel is in an optimum state (no appreciable friction) of pure rolling over the rail. However, because the rail and the wheel are profiled, misaligned and subject to motions other than strict rolling, the respective velocities at which the rail and the wheel move through the zone of contact are not the same and sliding occurs. The magnitude and the speed of the sliding is dependent on the difference, expressed in percentage, between the rail and wheel velocities at the point of contact. This percentage difference is termed creepage.
When creepage takes place, the entire zone of contact between wheel and rail is in a state of sliding and frictional forces are generated. Since most unlubricated "steel-on-steel" surfaces exhibit a negative friction characteristic (the frictional force decreases with the speed of sliding) the rail wheel interface is excited vibrationally and thus noise, in a wide range of frequencies, is produced. Because of the nature of rail vehicles and track structure, these systems cannot be made rigid so as to eliminate the noise generated by the creepage.
Reduction of friction to very low levels, at which the friction characteristic is flat (neither positive or negative) is not practical because a flat friction characteristic would prevent vehicles from having sufficient grip for braking or accelerating.
The oscillatory motion in a rail wheel interface, in addition to raising generated noise level, produces an undulatory wave pattern on the rail or wheel surfaces. With time, under such motion, wave-like patterns are formed onto the rail and wheel surfaces. These undulations are generally referred to as corrugations of a short-pitch (5 cm, 2"). A rail with corrugations is sometimes called a "roaring rail". When this occurs, the noise levels are increased beyond those for a smooth rail wheel interface.
Usual practice to cure this problem is to remove rail corrugations by grinding or machining the rail or wheel surface. The rail or wheel are smooth for a while but with use corrugations again build up over time. In addition such grinding of rails and wheels is time consuming and expensive.